1. Field of the Invention
The present invention relates to communications through a digital exchange network and more particularly to an information exchange control on multimedia communication carried out through a digital exchange network between a pair of subscribers using multiple pairs of terminal devices.
2. Description of the Prior Art
In the digital exchange network, each subscriber line is usually connected with a terminal device (hereafter referred to simply as a terminal) through a terminating equipment. The type of the terminating equipment varies depending on the data transmission rate of the terminal. The exchange system allocates to each terminal a predetermined time slot which corresponds to the terminal's accommodated location in the exchange system.
The Japanese Patent Laid-Open No. 33763/1985 discloses an improvement in which the number of time slots allocated to each terminal is dynamically controlled. In this method, the terminal specifies the transmission rate class through a control channel for example and the exchange system searches vacant time slots in a number corresponding to the specified transmission rate class and allocates them to the requesting terminal. This method permits a plurality of terminals with different transmission rates to be connected to terminating equipment through a switching circuit so that any one of these terminals can be used for communication. This method, however, does not consider multimedia communication in which a pair of subscribers exchanges information by simultaneously using two or more terminals accommodated in a single piece of terminating equipment. The I Series Recommendation (1985) of CCITT stipulates standards for the exchange system in performing exchange controls at the subchannel level. The above multimedia communication is considered feasible by using the exchange control at the subchannel level.
In realizing the above-mentioned subchannel exchange control on the exchange system side, there are two possible methods. The first method is that during the exchange process a call using the subchannel, too, is first assigned to the entire channel of the exchange data unit (say, 64 Kbps) and then returned to the original subchannel. The second method uses many subchannel switches provided in the channel. The first method, however, results in a large-scaled switching system which is poor in using efficiency. The second method also has the disadvantage of a complex switching system configuration.